Nowadays, rare earth magnets as typified by Nd magnets are widely used in various motors, sensors and other components mounted in hard disk drives, air conditioners, hybrid vehicles, and the like. As to the rare earth elements from which rare earth magnets are manufactured, their resources are found only in limited countries. A resource crisis is exclaimed because it is expected that the demand will exceed the supply in the near future. There is a strong demand for the reuse of magnet powder, debris and scraps associated with the manufacture of rare earth magnets, the recycling by recovery of rare earth elements from municipal wastes, and the research and development of new rare earth mineral deposits.
Most rare earth elements deposited in currently opened mines over the world are light rare earth elements (LREE) include lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd). Known techniques for the separation of rare earth elements include ion-exchange resins (solid-to-liquid extraction) and solvent extraction (liquid-to-liquid extraction). For the separation and purification of rare earth elements in an industrial manner, the solvent extraction technique is often used because consecutive steps enable efficient large volume processing.
In the solvent extraction technique, an aqueous phase in the form of an aqueous solution containing metal elements is contacted with an organic phase containing an extractant for a specific metal element and an organic solvent for diluting the extractant, for thereby extracting the specific metal element with the extractant. The specific metal element is separated in this way.
A variety of extractants are used in the art, for example, tributyl phosphate (TBP), carboxylic acid (Versatic acid 10), phosphoric acid esters, phosphonic acid and phosphinic acid. An exemplary phosphoric acid ester is di-2-ethylhexylphosphoric acid (D2EHPA). A typical phosphonic acid is 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester which is commercially available as PC-88A from Daihachi Chemical Industry Co., Ltd. A typical phosphinic acid is bis(2,4,4-trimethylpentyl)phosphinic acid which is commercially available as Cyanex 272 from American Cyanamid. They are commonly used in the industry.
The separation efficiency of the solvent extraction technique depends on the ability of an extractant, especially its separation factor. The greater the separation factor, the higher becomes the separation efficiency of the solvent extraction technique, which leads to simplification of separation steps and downsizing of the separation system, and eventually to improved process efficiency and a cost reduction. Inversely, if the separation factor is lower, the separation process becomes more complicated and the separation system becomes of larger size.
Of the commercially available extractants, even PC-88A which is known to have a high separation factor for rare earth elements has a low separation factor between neighboring elements in the Periodic Table, for example, a separation factor of less than 2, specifically about 1.4 between neodymium and praseodymium which are believed most difficult to separate among the rare earth elements. The separation factor of this order is not sufficient to facilitate separation between neodymium and praseodymium. To purify and separate one from the other at an acceptable purity, a large size system is necessary and capital-intensive. For the purpose of purifying and separating such is rare earth elements, it is desired to have an extractant having a higher separation factor than ever and an extraction/separation method using the same.
One extractant known to have a high separation factor for rare earth elements, especially LREEs: lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd) is a diglycol amic acid as disclosed in JP-A 2007-327085. Solvent extraction using this extractant makes more efficient the process of extracting and separating rare earth elements, especially LREEs. However, the extraction process must be further improved before extraction can be implemented at the industrially acceptable level because the diglycol amic acid has different chemical properties from the commercially available extractants, D2EHPA, PC-88A and Cyanex 272.